Neurons of the cerebral cortex are embedded in a highly interconnected network, allowing them to participate in a nearly infinite variety of neural circuits. The activity of a single neuron is determined by the flow of activity within this flexibl and powerful circuit. We hypothesize that neurons receive two broad categories of synaptic inputs that determine their action potential discharge: a slow, and sometimes, maintained component that strongly influences neuronal excitability and provides the cell with context; and a more rapid, temporally precise component that determines exactly when a cell discharges. The broad component is mediated by a proportional or balanced interaction of recurrent excitatory and inhibitory interactions in the cortex. The more rapid component appears to be mediated by temporally precise interactions of specific excitatory and inhibitory networks. Who are the interneurons that contribute to these two components and how are they achieving their task? Here we will address this important question through advanced in vitro and in vivo imaging and recording techniques. Using a recently developed in vitro mouse entorhinal cortical slice preparation that robustly generates spontaneous rhythmic Up and Down states, we will address the activity of several subclasses of neuron, including two excitatory cell types (Pyramidal and Stellate) and several inhibitory neuronal subtypes (PV/FS, SOM, NPY, 5HT3a, CR, VIP). This preparation not only generates the slow oscillation between Up and Down states, but also robust gamma-frequency oscillations during the Up state. Thus, we will also utilize this preparation to investigate the interneuronal cell types robustly involved in the generation of not only the general balance of recurrent excitation and inhibition that underlies persistent activity n the cortex, but also the generation of higher frequency network oscillations and the fast synaptic events that trigger action potentials on a precise time scale. These experiments will involve whole cell patch clamp and local field/MU recordings, as well as the selective inactivation/activation of known interneuronal cell populations with light activation of virally expressed archaerhodopsin, halorhodopsin, or channelrhodopsin. We will extend these investigations in vivo by studying the activity of identified subpopulations of interneurons in the mouse somatosensory cortex through patch clamp recording in a custom-built two-photon microscope. In a separate, but related, study, we will also investigate the possibility that local electrical fields may make a contribution to the synchronization and timing of higher frequency network oscillations. These investigations will help determine and define how the cerebral cortex operates, how it achieves a relative balance of recurrent excitation and inhibition, and how deviations from this balance may be critical in the generation of precisely timed action potential generation. The balanced operation of recurrent excitation and inhibition in the cortex is essential to the normal operation of the cerebral cortex and the breakdown of this balance readily results in the generation of epileptic seizures as well as psychiatric disorders.